Welding Processes: FCAW, GMAW, GTAW, SMAW, SAW

In process industry there are many pipes which are running across the plant transferring different fluid from source to the destination. These pipes are generally manufactured in a standard length of 6 metres or 12 meters so the pipes are joined together to achieve the required routing. The most common method of joining pipes is by welding two pipe spools together.

The most common weld processes for welding of piping are mentioned below with their requirements:

• Flux Core-Arc Welding (FCAW)
• Gas Metal-Arc Welding (GMAW)
• Gas Tungsten-Arc Welding (GTAW)
• Shielded Metal-Arc Welding (SMAW)
• Submerged-Arc Welding (SAW)
• Filler Metals, Electrodes, Wire and Flux  

1. Flux Core-Arc Welding (FCAW)

   1. Wind velocities shall not exceed 5 mph in the weld area.
   2. Gas shielded electrodes shall always be utilized, and the electrode diameter shall not exceed 1/8”.
   3. Vertical downhill welding shall not be permitted.
   4. Self-shielded FCAW shall not be permitted for process piping, but may be used for pipe support fabrication.
   5. Low hydrogen consumables shall be certified by the electrode Contractor, or tested by Contractor to result in less than 10 ml of diffusible hydrogen per 100 gm of deposited weld metal. Diffusible hydrogen content shall be measured by the mercury displacement or gas chromatograph method per AWS A4.3.  Contractor shall provide written evidence of compliance with this requirement.  The specification sheet for the proposed brand of filler metal from the Contractor shall be acceptable as written evidence.  

2. Gas Metal-Arc Welding (GMAW)

   1. Wind velocities shall not exceed 5 mph in the weld area.
   2. Short-arc GMAW shall be used only for root pass butt welding on piping.
   3. Short-arc GMAW shall not be used on weld-o-lets, reinforced nozzles and branch welds, couplings, slip-on flanges, socket welds or other pipe appurtenances.  

3. Gas Tungsten-Arc Welding (GTAW)

   1. Wind velocities shall not exceed 5 mph in the weld area.
   2. An internal gas purge shall be provided for the root pass and the second layer of autogenous and stainless steel welds.  

4. Shielded Metal-Arc Welding (SMAW)

   1. Wind velocities shall not exceed 10 mph in the weld area.
   2. Low hydrogen electrodes shall be required.
   3. If SMAW low hydrogen electrodes are unsuitable for producing acceptable radiograph quality, then GTAW, GMAW, or cellulosic SMAW electrodes shall be used on the root pass on single-sided butt welds.
   4. Cellulosic SMAW electrodes shall not be permitted for welds on API flanges. 

5. Submerged-Arc Welding (SAW)

   1. Wind velocities shall not exceed 10 mph in the weld area.
   2. All welding shall be multipass, with semi- or fully-automatic equipment.
   3. Weld passes shall not be greater than 3/8” thick.
   4. Addition of alloy agents through the flux shall not be acceptable.  

6. Filler Metals, Electrodes, Wire and Flux

   1. Electrodes, wires and fluxes shall be selected to produce welds with mechanical properties not less than those of the base metal. Filler metal for welding similar materials shall be of the same nominal analysis as the base metal, except E-347 electrodes shall be used for SMAW welding of Type 321 material.  When welding two steels of unequal strengths, the welding wire or electrode shall be matched to the higher strength material.

Additional requirements

1. Oxyacetylene welding shall not be permitted.
2. All semi-automatic or fully-automatic welds shall be made utilizing a multi-pass technique.
3. The weld filler metal in low-temperature piping service shall meet the same impact test requirements as the base metal.
4. Tensile strength of deposited weld metal shall be limited to 100 ksi maximum and shall be verified by a hardness test of the weld material in the procedure qualification. Weld metal hardness shall not exceed 200 Brinell.
5. Carbon steel electrodes for welding API 5L, Grades B and X-42 pipe shall conform to the following ASME specifications:
   

   SMAW	ASME SFA-5.1
   GTAW/GMAW	ASME SFA-5.18
   SAW	ASME SFA-5.17
   FCAW	ASME SFA-5.20 Classes: E60T-1, E60T-5, E61T-1, E61T-5 E70T-1, E70T-5, E71T-1, E71T-5

6. Stainless steel electrodes shall be low carbon “L” grade, and shall conform to the following ASME specifications:
   

   SMAW	ASME SFA-5.4
   GTAW/GMAW	ASME SFA-5.9
   FCAW-g	ASME SFA-5.22

7. Filler metals for joining carbon steels to stainless steels shall be E309L, except in services over 600°F where Inco 182 shall be used. Low carbon grades of austenitic stainless steel shall be joined with low carbon weld metal of corresponding composition.
8. The following SMAW SFA-5.1 electrode classifications shall not be acceptable for pressure containing welds:
   

   E6012

   E6013

   E6020

   E7014

   E7020

   E7024